Many types of communications require that a transmitter and receiver be synchronized. For example, in time-division multiplex transmission of independent digital bit streams, synchronization is necessary to correctly interpret the value of the incoming signal as one member of the transmitted symbol set and to direct the interpreted symbol to the correct receiver. Likewise, in television transmission, picture elements displayed at the receiver must be in the same relative positions as those appearing at the transmitter; and, for a line-and-field scanned television signal (one in which the picture being transmitted is scanned in fields of adjacent parallel lines), synchronization is necessary to achieve this.
The most common method of synchronizing television is by means of a horizontal synchronization pulse, of prescribed characteristics, in the horizontal blanking interval (HBI) of each line, and a set of vertical synchronization pulses, of different prescribed characteristics, in the vertical blanking intervals (VBI) between fields. In the receiver, these pulses are separated, from the picture information and from each other, and used to control the horizontal and vertical deflection oscillators which direct the picture tube's electron beam.
At times, however, it is desirable to eliminate at least the horizontal synchronization pulses from the television signal. This may be done in order to prevent reception of the signal (as in pay television) or to free the horizontal blanking interval for use in carrying other data, such as the audio portion of the television program. In either case, only the synchronization information transmitted in the vertical blanking interval is then available to synchronize the transmitter and receiver. A typical system uses a single synchronization pulse during the VBI and develops all control signals for deflection oscillators from this single synchronization pulse via counters actuated by an extremely accurate clock.
Where digital audio channels are multiplexed with the picture information during the horizontal blanking interval, a clock and data recovery system must exist in the receiver for the audio. This clock and data recovery system can also be used for synchronization recovery in the vertical blanking interval provided a digital synchronization word is used which employs the same clock. The simplest, and hence most desirable, method of synchronization would therefore be based on recovery of synchronization from a digital word in the vertical blanking interval.
Because of the absence of any line synchronization pulses in such a system, synchronization information is available only once each field. It is therefore extremely important that this infrequent synchronization information be reliably received, even in the presence of noise; for the instant of synchronization is the instant at which all system counters in the receiver are reset to zero. Failure to reset at the proper instant would perpetuate and magnify errors and eventually result in complete loss of the picture.